The surgical preparation of bone endings for receiving prosthetic knee joints for a total knee replacement is generally a complex procedure, particularly when ligaments remain attached, or when osteoarthritic changes to the joint have distorted the normal, more symmetric articulation geometry of the joint or bone. In general, it is necessary to perform soft tissue balancing and numerous specially aligned cuts at the bone ends in order to install the prosthetic components with correct spacing and alignment to prevent improper kinematics from arising as the joint rotates in use, and prevent the occurrence of accelerated wear patterns or possible joint dislocation.
A number of bone cuts are made to effect the placement and orientation of the femoral component of the prosthesis, and to determine and form the joint gaps in extension and flexion. The size and shape of these two gaps affect final orientation of the prosthesis, as well as joint tensioning and clearances. With respect to their effect on final orientation, the flexion gap is related to internal/external orientation of the femur, while the extension gap is related to the varus/valgus orientation of the femur.
Generally, these cuts are formed so that in extension the joint gap is perpendicular to the mechanical axis of the femur, while in flexion the joint gap is such as to place the femoral component in either neutral or external rotation and assure proper patellar tracking with the femoral component. Furthermore to fit the femoral component the gaps created by the bone resections in both flexion and extension should be rectangular.
Typically this requires a number of measurement steps and cutting or fitting steps, often with additional small adjustment cuts to achieve the fmal bone preparation. However it is difficult to devise a jig which dependably sets the degree of femoral rotation, because landmarks may be inconsistent or obscure. In general, the surgeon must exercise judgment as the various cuts are made. Also the steps in reaching a determination will vary depending upon the initial landmarks used for setting preliminary resections, both as a matter of the surgeon's preferred procedure and as constrained by any patient-specific features or disease.
Accordingly, it would be desirable to provide a tool to simplify procedures for performing preparatory bone cuts or markings for preparing the bone to receive a prosthetic joint component.